An electroglottograph (EGG) is a known biosensor having electrodes which contact the subject's neck. The EGG supplies a constant rms current to the neck, e.g. 5 mA, at a frequency of about 5 MHz. When the subject speaks or sings there is a corresponding change in the impedance of the neck at the frequency of the sound being uttered, which change is detected by the electrode as a change in voltage. The EGG, using, e.g., a Wheatstone or Fabre bridge circuit, detects and measures this change in impedance in relation to the frequency of the speech. The EGG provides an accurate waveform of the subject's voice through a full range of frequencies.
The output from a microphone which picks up the sound concurrent with the same vibrations as sensed by the the electrodes may also be applied to the EGG to indicate changes in volume. The EGG has been used for medical diagnosis and speech therapy purposes only. A known EGG device is described in U.S. Pat. No. 4,139,732 to Fourcin, the contents of which are incorporated herein by reference.
Voice-activated musical synthesizers are also known in the art. U.S. Pat. No. 4,771,671 to Hoff and U.S. Pat. No. 4,463,650 to Rupert disclose voice controlled musical devices which use a microphone as an input. U.S. Pat. No. 4,193,332 to Richardson discloses a music synthesizer which receives an input from an instrument or a transducer. Fairlight Instruments Pty. Ltd. of Australia makes and sells "The Voicetracer" which provides real-time synthesizer response to a signal from a microphone.
These prior art devices have not worked well because of the well known difficulties in extracting pitch information from microphone input. Even with cost and time prohibitive amounts of computer processing, pitch extractions are not reliable over a wide range of voicing behaviors and especially in the vicinity of voice onset or release. The physical reasons are 1) the vocal tract filters the glottal source in complex ways that depend on vowel choice, 2) the vocal tract is often configured as a bandpass filter at a frequency above the fundamental, 3) the mouth differentiates the output of the vocal tract, thus further supressing low frequency information, 4) often, especially at voice onset and release, spectral considerations implied by comments 1-3 are irrelevant, and a pulse-by-pulse model is necessary, and 5) the energy from the mouth varies with vowel, microphone position, emphatic segmentation, etc., and furthermore, it is easlily contaminated by ambient sound, the most serious contamination being the matched sound of the synthesizer under control. For these and many other reasons, pitch extraction from a microphone is only marginally useful and has generally failed to meet the extremely demanding requirements of a musical device.